1. Field of the Invention
The field of the present invention is electronic amplifier circuits. More particularly, the present invention relates to a variable gain amplifier circuit.
2. Description of the Related Art
Wireless devices are transforming how people work, relax, and communicate. These devices can enable convenient access to informational, educational, and entertainment data, and provide a convenient portal for worldwide communication. Some of the most popular wireless devices are portable, which benefit not only from a small footprint, but also require a consistent and robust communication link to be useful. Without the benefit of such a dependable communication link, users are unable to reasonably rely on the availability of their wireless devices.
Generally, a wireless device has a radio transceiver that communicates with other mobile devices or to a more permanent base station. Accordingly, the wireless device has an antenna that is used to both transmit and receive radio frequency signals. In particular, the antenna and wireless device are typically configured to operate on a particular range of radio frequencies, with an information signal modulated on the radio wave.
It is a particularly difficult problem to configure a wireless device to reliably and robustly receive signals in a manner that enables the information signal to be consistently demodulated and used. Several factors affect the quality of reception and the usability of the information signal. For example, the modulation signal may be subjected to physical interferences, such as buildings, that substantially attenuate the modulation signal. Further, distance from the modulation signal source also substantially attenuate the modulation signal. To facilitate receiving an attenuated signal, the wireless device will often attempt to amplify the modulated signal to a usable level. However, such amplification not only amplifies the modulated signal and information signal, but also amplifies noise and disturbances. Accordingly, the signal to noise ratio of the modulated signal deteriorates until the information signal is indiscernible from the noise.
Amplifiers used in wireless devices often operate in a substantially linear fashion. This is particularly true for the receiver section where any non-linearities introduced during processing of the incoming signal can produce distortion products that will interfere with the accurate reception and demodulation of the incoming information signal. Such distortion products will produce errors within the received signal information. In wireless applications, the receiver section must typically deal with a wide range of incoming signal levels and such a wide range can create some conflicting requirements for the amplifier. For example, for weak signals the amplifier should provide significant gain, often in the range of 12-16 decibels (dB) but do so without adding excessive noise to the signal. This helps preserve the signal-to-noise ratio (SNR) through the receiver section and allows the received signal to be recovered without errors due to excessive noise. However, because the amplifier is providing such significant gain, stronger received signals can easily overload the amplifier or subsequent stages within the receiver section and thereby cause significant errors because of distortion products.
Implementing an amplifier that can handle the largest received signal without distortion requires the amplifier to be capable of a high-power output. This is generally in direct conflict with a requirement for low power consumption that is necessary for hand-held devices which are typically powered by batteries. (The noise level is less problematic in the presence of a strong signal where the strength of the received signal is such that it can be recovered notwithstanding the noise level. In these instances, noise performance can be sacrificed for improved linearity.)
The popularity and interoperability of wireless devices is enhanced by communications standards. Accordingly, most wireless devices adhere to one of the handful of common communications standards. Some of these standards, such as the widely popular CDMA (Code Division Multiple Access) standard, benefit from a variable gain amplifier. Indeed, in CDMA, for example, the gain of the transmitter amplifier is continually adjusted to the minimum operating power. Accordingly, the wireless device is often receiving only a very small modulation signal. Although this provides superior channel separation in a CDMA system, the amplifier arrangement must be configured to a high sensitivity. However, such a setting may leave the wireless device susceptible to spikes, surges or other forms of large changes in the modulation signal. If a large signal causes the wireless device to saturate, then the wireless device may drop a communication or be damaged.
For the best signal recovery in CDMA wireless receiver devices, the receiver section characteristics are modified as the signal levels change within the received signal. The ability to receive weak signals without adding excessive noise and the ability to receive strong signals can be managed by controlling the characteristics of the first amplifier following the antenna within the receiver section. If such amplifier has a suitable, variable attenuation device built into the amplifier circuit, then the extremes of signal strength can be accommodated. For example, with such attenuation set to zero, the gain of the section is maximized as is the ability to receive weak signals. For strong signal levels, such attenuation must be increased above zero, thereby reducing the effective gain of the amplifier section. However, this has the advantageous effect of reducing the maximum signal level that the amplifier section, as well as subsequent stages, must process. As a result, the effective signal range in which the amplifier remains operating in a linear fashion is increased, with respect to the input port to the receiver section.
In achieving the best performance characteristics when processing small signals as well as large signals, such a variable attenuator, as part of the amplifier circuitry, will be required to have a number of characteristics. When the attenuator is set for zero attenuation, it should function as a xe2x80x9cstraight throughxe2x80x9d device, i.e., it should introduce no signal loss and should not adversely affect the noise performance of the amplifier section (e.g., by loading the input port of the amplifier and thereby changing its input impedance).
Additionally, when the attenuator is set to provide some amount of signal attenuation, it should have no adverse effect on the operation of the amplifying device. Such adverse effects would include changes introduced to the biasing conditions of the amplifier or the introduction of some signal distortion characteristic that is not otherwise present during operation of the amplifier.
Further, the attenuator should provide effective attenuation at the input of the amplifier so that the maximum output required at the output of the amplifier is sufficiently reduced when such attenuation is provided. Moreover, the attenuator should introduce no excess noise so as to exacerbate the degradation in SNR that is otherwise inherently introduced with attenuation, nor should the attenuator introduce any significant non-linear operating characteristics during operation at any of its levels of attenuation.
An amplifier circuit with shunt and feedback impedance circuits for providing a controllable variable signal gain in accordance with one embodiment of the present invention includes: an input signal terminal; an output signal terminal; a signal reference terminal; a shunt impedance circuit; a feedback impedance circuit; and an amplifier circuit. The input signal terminal is for conveying an input signal having an input signal magnitude and phase. The output signal terminal is for conveying an output signal having an output signal magnitude and phase. The signal reference terminal is for establishing a signal reference node. The shunt impedance circuit is coupled between the input signal terminal and the signal reference terminal and has a shunt impedance that varies in response to reception of a control signal. The feedback impedance circuit is coupled between the input and output signal terminals and has a feedback impedance that varies in response to reception of the control signal. The amplifier circuit, coupled between the input and output signal terminals, has an associated amplifier signal gain and provides the output signal in response to reception of the input signal, wherein the output signal phase is inverse to the input signal phase and a ratio of the output and input signal magnitudes varies in relation to a circuit signal gain. The amplifier signal gain, the shunt impedance and the feedback impedance together determine the circuit signal gain.